In brain, p42(IP4) (centaurin-alpha1; recently named ADAP 1 within the large family of Arf-GAP proteins) is mainly expressed in neurons. p42(IP4) operates as a dual receptor recognising two second messengers, the soluble inositol(1,3,4,5)tetrakisphosphate and the lipid phosphatidylinositol(3,4,5)trisphosphate. We show here for the first time that p42(IP4) is localized in mitochondria, isolated from rat brain and from cells transfected with p42(IP4). In rat brain mitochondria we additionally found interaction of p42(IP4) with 2', 3'-cyclic nucleotide 3'-phosphodiesterase and alpha-tubulin by pull-down binding assay and by immunoprecipitation. In mitochondria from chinese hamster ovary cells, p42(IP4) is predominantly associated with the intermembrane space and the inner membrane. This localization of p42(IP4) indicates that p42(IP4) might have a still unknown mitochondrial function. We studied whether p42(IP4) is involved in Ca(2+)-induced permeability transition pore (PTP) opening, which is important in mitochondrial events leading to programmed cell death. We used mouse neuroblastoma N2a cells as a model for the functional studies of p42(IP4) in mitochondria. In mitochondria isolated from p42(IP4-)transfected N2a cells, overexpression of p42(IP4) significantly decreased Ca(2+)capacity and lag time for Ca(2+) retention. Thus, we suggest that p42(IP4) is involved in the regulation of Ca(2+) transport in mitochondria. We propose that p42(IP4) promotes Ca(2+)-induced PTP opening and thus destabilizes mitochondria.